γ-carotene

γ-Carotene (gamma-carotene) is a carotenoid, and is a biosynthetic intermediate for cyclized carotenoid synthesis in plants. It is formed from cyclization of lycopene by lycopene cyclase epsilon. Along with several other carotenoids, γ-carotene is a vitamer of vitamin A in herbivores and omnivores. Carotenoids with a cyclized, beta-ionone ring can be converted to vitamin A, also known as retinol, by the enzyme beta-carotene 15,15'-dioxygenase; however, the bioconversion of γ-carotene to retinol has not been well-characterized. γ-Carotene has tentatively been identified as a biomarker for green and purple sulfur bacteria in a sample from the 1.640 ± 0.003-Gyr-old Barney Creek Formation in Northern Australia which comprises marine sediments. Tentative discovery of γ-carotene in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is significant for reconstructing past oceanic conditions, but so far γ-carotene has only been potentially identified in the one measured sample.

== Background == γ-Carotene is a carotenoid, a class of pigments giving color to photosynthetic organisms. Specifically, γ-carotene may be derived from myxoxanthophyll found in cyanobacteria, Chlorobiaceae, and green non-sulfur bacteria (Chloroflexi). However, there are over 600 different carotenoids, each with different structures and formulas thus altering their absorption spectrum. In particular, Chromatiaceae lie between 1.5 and 24 meters deep into the water column with more than 75% of the microbial blooms occurring above 12 meters deep. Other carotenoids such as chlorobactane and isorenieratene are also biomarkers for the presence of green non-sulfur bacteria. These carotenoids are indicators of the past aquatic geochemical environment of their source water. In particular, γ-carotene is an indicator of the depth at which oxic conditions move towards anoxic conditions due to its relevance to green and purple sulfur bacteria which occupy the boundary layer. Green non-sulfur bacteria are known to produce 2,3,6-trimethylaryl isoprenoids which are unambiguous, thus permitting the deduction of past aquatic geochemical environments. In γ-carotene, the end group of lycopene produces a β-ring via a β-cyclase enzyme. The other end member is attributed to an open-chain ψ-end.